Finding disease-modifying treatments for Parkinson's disease (PD) requires new animal models that faithfully reproduce its cardinal biochemical and clinical features. This application describes a novel mouse model that has unique advantages over published PD mice. It is based on destabilizing a previously unrecognized physiological form of ?-synuclein (?S): ?-helical tetramers (T), which are in equilibrium with the long-studied natively unfolded monomers (M). Since their discovery in 2011, ?S tetramers have been observed by a dozen labs, but their relationship to PD and DLB remain controversial. ?S tetramers resist aggregation and ?- and ?- synucleins likewise form tetramers, underscoring their physiological nature. Given ample evidence that ?S is the key misfolded protein in `sporadic' PD and is implicated in familial PD, including in GBA and LRRK2 mutant carriers, it is crucial to elucidate how ?S converts from a normal ?-helical to an aggregate-prone ?-sheet form and to interfere with this. In cultured neurons, all PD-causing ?S missense mutations shift T to excess free M prone to aggregation. Critically, another lab just published that GBA-mutant human neurons from Gaucher's carriers have decreased T:M ratios of their endogenous wt ?S, and genetic or pharmacological restoration of GBA corrects this (Kim, PNAS 2018). This discovery strongly underpins the new work proposed by us. Our central hypothesis is that homeostasis requires maintaining a normal T:M equilibrium, whereas excess monomers may initiate PD and DLB. To this end, the PI generated unique transgenic mice that express GluLys ?S mutants which amplify the E46K fPD mutation and cannot form tetramers, leading to an age- dependent motor phenotype strikingly similar to PD. ?S monomers accumulate at vesicle membranes and form round inclusions in cell bodies and neurites. ?S becomes insoluble, proteinase K-resistant and Ser129 phosphorylated, all features of ?S in PD. These changes affect regions critical for motor behavior, including nigrostriatal dopaminergic neurons that develop lysosome- and lipid-rich lesions. The outcome is a delayed, progressive PD-like motor syndrome affecting males more than females and including resting tremor, gait and limb deficits partly responsive to L-DOPA. This fully penetrant phenotype means that tetramers are required for the normal state of ?S in dopamine & other neurons and suggests that shifting T to M can initiate PD. Our new work will exploit this mouse model of PD. Aim 1 compares this 3K mouse to E46K (?1K?) mouse to see if 3K quantitatively exaggerates the qualitative phenotype of a ?true fPD? model. Aim 2 takes advantage of the PD- like gender disparity to ask a little-studied question: which biological factors offer relative PD protection to females. And based on striking prelim. data, Aim 3 proposes 2 new in vivo treatment approaches: elevating brain-estrogen in 3K males to explore pathways that protect from fiber loss and ?S-aggregation; and inhibiting SCD-1, which prevents the T:M shift and associated Lewy-like inclusions and toxicity in culture. Thus, my first R01 application phenotypes an entirely novel mouse model, with attendant therapeutic opportunities. !